The activity of catechol-O-methyltransferase (COMT) was investigated in cultured and propagated cerebromicrovascular endothelial and smooth muscle cells using high performance liquid chromatography and immunocytochemistry. The existence of COMT was detected in both cell types. The demonstration of this enzyme activity in the cerebromicrovascular smooth muscle cells, in addition to the endothelium, indicates that the enzymatic barrier to catecholamine is not limited to capillaries, the main constituents of the blood-brain barrier.